Genes Matter

The probability of having a boy or a girl according to genetics

Did you know that the probability of having a boy or a girl is determined by genetics? After all, foetal sex depends on whether the sperm cell carries an X or Y sex chromosome. Do you want to know why? It’s all in this article, where we also talk to you about the new methods for finding out the sex of your baby before your 16-week ultrasound scan. 

What is the genetic difference between a female and a male embryo?

We have already seen this in articles such as What is DNA?, in each of our cells, human beings have 23 pairs of chromosomes (overall, 46 chromosomes).

The first 22 pairs of chromosomes are called “autosomal” and are the same in both sexes. 

However, the chromosomes of pair 23 are different in males and females and are called sex chromosomes: 

  • Pair 23 chromosomes in women: XX.
  • Pair 23 chromosomes in males: XY.

The probability of having a boy or a girl according to genetics 

Firstly, we should talk a little bit about gametogenesis, which is the process in which sex cells are generated. That is, sperm in the case of men, and eggs in women.

These sex cells, i.e. the egg and sperm cells, do not contain 23 pairs of chromosomes like the rest of the cells in the body, but have 23 chromosomes each. Therefore, when a sperm cell fertilises an egg, they form a zygote which will have all 46 chromosomes, 23 from the mother and 23 from the father.

Eggs always have an X sex chromosome because, as explained above, women have two X chromosomes. Sperm, however, can either have an X or a Y chromosome. Therefore, if a sperm with an X chromosome fertilises an egg, the resulting zygote will be female (XX). On the other hand, if the sperm which fertilises the egg has a Y chromosome, the zygote will be male (XY). In short, genetically, the probability of having a boy or a girl depends on whether the sperm that fertilises the egg is X or Y.

What factors can influence this process?

Now that you know how the sex of human beings is determined, you might be wondering if there are factors that influence whether the sperm that reaches the egg is X or Y.

Well, even though it is a random process, some studies suggest that the probability of having a boy or a girl is influenced by the day that intercourse takes place in relation to ovulation. 

This is because sperm cells with an X and Y copy have some different characteristics: 

  • Y sperm: are faster but survive for less time in the female genital tract. 
  • X sperm: are slower, but they are more resistant and therefore survive longer.  

Can you “choose” the sex of your baby? 

Considering the facts mentioned above, there are two hypotheses that can influence the probability of having a boy or a girl:

  • Speed: since sperm containing the Y chromosome are faster than those containing the X chromosome, if the egg is in the fallopian tube at the time of intercourse, there is a greater probability of a Y sperm reaching and fertilising it, which means there is a greater probability of the baby being a boy. 
  • Longevity: since sperm containing the X chromosome are able to live longer in the female genital tract, if the egg is not in the fallopian tube at the time of intercourse, sperm with an X chromosome will be able to survive longer until the egg reaches the fallopian tube. This therefore increase the chance of the baby being a girl.

In short, these two hypotheses explain that if intercourse takes place when ovulation has already occurred, the Y-copy chromosomes will reach the egg sooner and the chance of having a boy will be greater, whereas if it takes place before ovulation this will increase the chances of having a girl. This is because X-copy chromosomes can survive for longer while the egg reaches the fallopian tubes.

However, don’t forget that these are only hypotheses, and even if you know when you are ovulating (which you can find out using an ovulation test) and you decide to try these methods, there is no guarantee that the baby will actually be one sex or the other.

How to find out the sex of your baby?

The curiosity to know the sex of your baby when you are expecting is very common and dates back to historical times. When we look as far back as Greek medicine, in the medical writings Corpus Hipocraticum, different characteristics of pregnant women were described to help determine the sex of the baby before birth. One of them was based on a woman’s breast positioning, indicating that if they were turned downwards the woman was expecting a boy and if they were turned upwards it was a girl.

There are also popular beliefs that have been passed down to us, such as the one that predicts the sex of the baby based on the roundness of a pregnant woman’s bump. This belief suggests that if the bump is more pointed, it is a boy, and if it is rounder, it is a girl.

Remember that these theories have no scientific validity, but thanks to modern scientific advances, we can accurately and safely determine the sex of the baby in the first few weeks of pregnancy.

The ultrasound method

There there is already an outline of the foetus’s genitalia at 12 weeks. Therefore, as long as the baby is in the correct position, the sex can be detected by an ultrasound scan, with only a 20% margin of error.

Confirmation of sex happens at week 16, when the male and female genitals are now clearly distinguishable. But we must not forget that we can only confirm this if the child is in the correct position for us to observe the genitals.

The Ramzi Theory

There is not 100% scientific validity to this method. However, there is data published that correctly corroborates the sex of the foetus in 97.2% of male foetal sex cases and 97.5% of female foetal sex cases. It relies on the different positioning of the placenta and chorionic villi in both sexes and can be used from the sixth week of pregnancy. 

New technologies to find out the sex of your baby

Did you know that it is possible to accurately find out the sex of your baby before the 16-week ultrasound?

As we explained in detail in the article on cell-free foetal DNA in maternal blood, we can detect fragments of DNA from placental cells in the mother’s bloodstream from the sixth week of pregnancy onwards.  To be able to test with an adequate concentration of cell-free foetal DNA, it is recommended to wait until week 10. 

This has been a breakthrough discovery, as it allows us find out if the baby has any chromosomal abnormalities with a non-invasive screening test that uses a sample from the mother’s blood.

In addition to detecting possible chromosomal abnormalities in the baby, it is also possible to determine the foetal sex and therefore see if there are any abnormalities in the sex chromosomes. 

Sex chromosome disorders 

This type of disorder occurs when there is an abnormal number of sex chromosomes in the baby. 

Among these abnormalities, which can result in physical, learning and infertility problems, the most common are Turner syndrome (only affects females and occurs due to the lack of a total or partial X chromosome), Klinefelter syndrome (only affects males and occurs because they have two X and one Y chromosome), Triple X syndrome (only affects females and occurs because they have three X chromosomes) or XYY syndrome (only affects males who have one X and two Y chromosomes).

How the screening test determines sex 

This test analyses the DNA of the placental cells. Detection of a Y chromosome is consistent with the presence of a male foetus. Therefore, if no Y chromosome fragment is detected, the inferred sex is female. 

In cases of twin pregnancies, if there is a Y chromosome present, we can determine that at least one of the foetuses is male, and if there is no Y chromosome, we can assume that both foetuses are female.

Tests to find out the sex of your baby and take care of their health 

Now you know why the probability of having a boy or a girl depends on genetics, and you are also familiar with the tests that inform you about possible chromosomal pathologies, as well as provide you with all of this information in the early weeks of pregnancy.

Would you like to know the sex of your baby as soon as possible? Are you curious to know if he or she might develop any diseases? 

It’s possible to find out with Veritas’ myPrenatal test, a non-invasive test designed to rule out chromosomal abnormalities such as Down’s syndrome, and to find out whether you are expecting a boy or a girl earlier than normal.

With the myNewborn test, you can find out if your baby has any actionable diseases postpartum that are not detected by the heel prick test. This is highly valuable information that allows you to take preventative measures or decide on the most appropriate treatment of a possible disease, subsequently preventing more serious consequences.

At Veritas we know that your baby’s health is the most important thing to you, which is why we have both of these tests available to you. If you want to know more, do not hesitate to contact us. We will be happy to explain all the benefits they bring for the well-being of your children, in detail.

See all

Maybe you may be interested

Leave a comment



7 January, 2022 2:54 pm

I went intercourse on ovulation day after 12 hrs of ovulation…even though my blood work said that am carrying female foetus….how it will be possible according to above theory


30 June, 2022 2:53 pm

i am very happy for this information that i got here was very help full


19 September, 2022 1:50 pm

Much apricated. Keep sharing more posts like this . Please Visit More Information.. Best IVF Center in Karachi

Yunusa sunday

12 April, 2023 4:49 pm

Very interesting

Modress Nchimunya

11 January, 2024 5:12 am

thank you, it helped me alot